314:, Utah, US) grow by ions extending the lattices of the radial crystals. The mode of growth of ooids with tangential (usually minute needle-like) crystals is less clear. They may be accumulated in a "snowball" fashion from tiny crystals in the sediment or water, or they may crystallize in place on the ooid surface. A hypothesis of growth by accretion (like a snowball) from the polymineralic sediment of fine aragonite, high-magnesium calcite (HMC) and low-magnesium calcite (LMC), must explain how only aragonite needles are added to the ooid cortex. Both in tangential and in radial ooids, the cortex is composed of many very fine increments of growth. Some modern (and ancient) ooids partially or totally lack clear layering and have a micritic (very fine grained) texture. Examination of such micritic ooids by scanning electron microscopy often shows evidence of microbial borings later filled by fine cement.
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An ooid forms as a series of concentric layers around a nucleus. The layers contain crystals arranged radially, tangentially or randomly. The nucleus can be a shell fragment, quartz grain or any other small fragment. Most modern ooids are
358:(Wilson et al., 2021). The formation of the ooid cortex around the test or shell protects it from abrasion, fragmentation and bioerosion. Ooimmuration also retains fine organic remains that would ordinarily be winnowed away by currents.
258:(or the aragonite layers in originally bimineralic ooids). Moldic ooids (or molds later filled in by calcite cement) occur in both young and ancient rocks, indicating the removal of a soluble polymorph (usually aragonite).
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Wilson, M.A., Cooke, A.M., Judge, S.A. and Palmer, T.J. 2021. Ooimmuration: Enhanced fossil preservation by ooids, with examples from the Middle
Jurassic of southwestern Utah, USA.
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are similar to ooids, but are larger than 2 mm in diameter, often considerably larger, as with the pisoids in the hot springs at
Carlsbad (
250:, and some are bimineralic (layers of calcite and aragonite). Ancient ooids can be calcitic, either originally precipitated as calcite (as in
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of the water with respect to calcium carbonate, the availability of nuclei, agitation of the ooids, water depth, and the role of
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levels, which in turn facilitate the substitution. Marine calcitic ooids were typically formed during
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and fluctuating Mg/Ca ratios. Low Mg/Ca ratios favor the precipitation of low-magnesium calcite.
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Fossils ooimmured in ooids from the Carmel
Formation (Middle Jurassic of southern Utah).
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Microfacies of
Carbonate Rocks: Analysis, Interpretation and Application
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A thin slice of calcitic ooids from the Carmel
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Ooids with radial crystals (such as the aragonitic ooids in the
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Small sedimentary grain that forms on shallow tropical seabeds
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Whether ooids become calcitic or aragonitic can be linked to
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Periods. The geochemistry of these seas was a function of
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of calcium carbonate; some are composed of high-magnesium
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are included in ooids, often forming the nuclei. This
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322:There are several factors that affect ooid growth:
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46:but its sources remain unclear because it lacks
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393:https://doi.org/10.2110/palo.2021.036
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32:This article includes a
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384:. Accessed 2014-06-23.
61:more precise citations.
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366:Flügel, Erik (2010),
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276:crystalline structure
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412:"Definition of OOID"
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280:marine environments
354:process is termed
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34:list of references
485:Sedimentary rocks
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53:Please help
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372:, 2nd ed.,
306:Growth mode
288:calcite sea
177:sedimentary
158:, from
59:introducing
479:Categories
421:2024-08-19
398:References
362:References
352:taphonomic
346:Sometimes
292:Ordovician
173:spheroidal
495:Petrology
490:Limestone
268:strontium
262:Variation
244:polymorph
240:aragonite
233:Formation
225:) in the
215:limestone
193:sea floor
189:phosphate
67:June 2014
374:Springer
328:microbes
296:Jurassic
294:and the
284:salinity
205:sediment
123:Jurassic
111:Jurassic
109:(Middle
467:at the
389:Palaios
348:fossils
272:calcium
248:calcite
219:Pisoids
197:Bahamas
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210:oolite
169:(ōión)
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130:Ooids
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378:ISBN
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